This invention relates to an improvement in the method of producing electrodeposits of tin, and is particularly concerned with a plating bath from which tin deposits can be obtained which are substantially immune to the effects of "tin pest".
It has been found that when materials bearing an electrodeposit of tin are maintained under conditions wherein the ambient temperature is less than 18.degree. C., the transformation of white (beta) tin of tetragonal form to the gray (alpha) tin of cubic centered form occurs. This transformation is commonly knonw as tin "disease" or "tin pest". The result of the transformation is that the specific gravity of the tin or tin electroplate is lowered from about 7.3 to about 5.75 with its attendant destruction of the tin's metallic properties. As the tin pest develops, a loose tin powder is formed which easily separates from the base metal. The tin then loses its ability to protect the base metal. Accordingly, the base metal is more susceptible to the effects of corrosion.
For many years it has been known that a small amount of bismuth in tin or tin electroplate inhibits the formation of the tin pest. It has been reported that a concentration of bismuth above 0.12 percent in tin or tin alloy inhibits and may even prevent the formation of tin pest at temperatures as low as -73.degree. C. for indefinite periods of time. MacIntosh, R. M. "The Properties of Tin at Low Temperatures," Tin Research Institute, Columbus, Ohio (Oct. 15, 1953). In the case of tin electroplate formed by an alkaline process, it is desirable to limit the concentration of bismuth to below 2 percent to avoid the formation of dendrites as well as grainy and very gray tin plates.
In U.S. Pat. No. 3,360,446 bismuth esters (i.e., hydroxy alkyl bismuthates, preferably polyhydroxy alkyl bismuthates) are disclosed for incorporation in alkaline potassium or sodium stannate tin plating baths in a concentration of from 0.05 to 1.5 grams/liter (expressed in terms of the bismuth metal content) to achieve a bismuth content of from 0.1-0.6% in the electrotinplate. Since the alkaline primary reaction mixture containing the bismuthate ester must be neutralized with a weak organic acid immediately to prevent decomposition and the neutralized product must be precipitated with an alcohol several times to be purified, the long term stability of the bismuthate ester in hot alkaline electrotinplating baths may not be very high.
U.S. Pat. No. 3,522,155 discloses the use of alkali metal bismuthates, particularly sodium bismuthates, in alkaline potassium or sodium stannate tin plating baths in a concentration of from 0.05 to 1.0 grams/liter (expressed in terms of the alkali metal bismuthate) for a range of 0.1-0.6% bismuth in the electrotinplate. Laboratory investigations have shown that sodium bismuthate has very low solubility in hot alkaline potassium stannate tin plating baths and tends to hydrolyze and precipitate.
The low solubility of alkali metal bismuthates in alkaline electrotinplating baths normally necessitates the use of a supersaturated solution of the alkali metal bismuthate. Several disadvantages exist with respect to the use of such supersaturated solutions in electrotinplating. Large amounts of the alkali metal bismuthate are required to place the required amount of bismuth into solution. In addition, the quality of the electrotinplate may be affected by suspended alkali metal bismuthate and hydrolyzed bismuth stannate precipitates. Such precipitates would promote sludging in the plating bath since they are autocatalytic to the formation of hydrolyzed stannic hydroxide precipitates. Thus the bath would require purification or replacement more often.